I can add -# Set the correct locale - current_page.add_metadata options: { locale: "fr-be", lang: "fr-be" } But this seems to only work after refreshing the said tag page - and does not build correctly either.

I believe that Middleman rebuilds or, at the very least, partially updates the metadata of resources when files.changed is fired, which is why the metadata set might be getting cleared. When you say it is unstable, what are the issues you are having?...